Add support for detecting SLNX solution files

[atifaziz]

This PR fixes #75.

dotnet-releaser new auto-discovery only looked for .sln files and missed the newer .slnx format. The underlying solution parsing library (SolutionSerializers) already supports .slnx, so only the discovery logic and user-facing strings needed updating with this PR.

When both .sln and .slnx exist in the same folder, .slnx is preferred since it is the newer format.

For verification, run these commands:

git clone --depth 1 --branch slnx https://github.com/atifaziz/dotnet-releaser.git
cd dotnet-releaser
dotnet build src
git fetch --depth 1 https://github.com/atifaziz/CSharpMinifier.git 6d8016cd58e37b1d37c195e3100c3208985066e1
git read-tree --prefix CSharpMinifier 6d8016cd58e37b1d37c195e3100c3208985066e1
git checkout .
git reset
cd CSharpMinifier
dotnet ../src/dotnet-releaser/bin/Debug/net10.0/dotnet-releaser.dll new

The last command should succeed with the following output:

2026/02/10 18:17:51.946 info: [1] Solution file detected: CSharpMinifier.slnx
2026/02/10 18:17:52.008 info: [2] git user/repo detected: atifaziz/dotnet-releaser
2026/02/10 18:17:52.014 info: [3] New configuration file `T:\dotnet-releaser\CSharpMinifier\dotnet-releaser.toml` created
successfully.

---

PS I noticed that commit 58d696a has the message “Add support for slnx. Switch to net9.0”, but I didn't see any such support for SLNX. For example, git grep -Fi slnx 58d696a51b0805299be9a0dc6813a26cbda662f4 gives nothing.

[xoofx]

Thank you! Indeed, I was in a rush when I had to patch dotnet-releaser to support slnx and forgot to update several stuffs.